Lead and zinc are invariably found together in nature. For example, the ore body located at Mount Isa, Queensland, Australia includes ore that contains both lead and zinc.
The lead/zinc ore at Mount Isa is mined and formed into two concentrates, a lead and a zinc concentrate. Separation between lead and zinc is not perfect, so some zinc reports to the lead concentrate and vice versa. The lead concentrate is sent to a lead smelter. The lead smelter converts the concentrate into lead metal and slag. The slag contains various fluxing agents used in the lead smelting operations and a significant quantity of zinc. The zinc content of the slag may vary between 10 to 15% by weight. The slag at Mount Isa has an average zinc content of around 12% by weight. The slag also contains significant quantities of iron, silica and calcium oxide. For example, the slag may comprise around 23% by weight iron, around 21% by weight silica and around 23% by weight calcium oxide.
As the slag derived from the lead smelting operations contains significant quantities of zinc, it represents a potentially valuable resource from which zinc can be recovered. Significant efforts have been expended over the previous 40 years to try to develop a process to recover the zinc from the slag derived from the lead smelting process. The most commonly used is the slag fuming, producing a zinc oxide that requires further treatment to remove deleterious impurities.
One problem encountered in previous efforts to recover zinc by hydrometallurgy from the lead smelter slag arises due to the presence of large amounts of silica (typically in excess of 20% by weight) in the slag. If the slag is subjected to a leaching step to leach the zinc from the slag using sulphuric acid as the leaching agent, silica is also dissolved in the leaching step. However, the dissolved silica then tends to form insoluble compounds that are in the form a gel which is extremely difficult, if not impossible, to separate from the liquor.
Another problem relates to the recovery of zinc from solution into a saleable form. The industry accepted way to achieve this is by removing all contaminants from solution by solvent extraction or purification and electrowinning to produce a relatively pure zinc product. This method to recover zinc is not possible in some regions, such as Mount Isa in Queensland as there is no access to sufficient electricity generating capacity for electrowinning. As a result, processes to precipitate the zinc as an oxide concentrate have been adopted in some instances. For example, the process described in U.S. Pat. No. 6,726,889 (the entire contents of which are herein incorporated by cross reference) has been trialled successfully to recover a high grade zinc oxide concentrate. In U.S. Pat. No. 6,726,889 a process is described where zinc sulphate solution is contacted with hydrated lime at pH of preferably from 8.5-9.5, typically at pH of 9.5, at a minimum 75° C., typically 90° C., to produce a zinc oxide and gypsum. The zinc sulphate solution contains zinc, silica and iron. The two products (zinc oxide and gypsum) can be separated gravimetrically and separation is improved by growing the gypsum through seed recycle. When a split size at approximately 30 μm is achieved, a high quality zinc oxide concentrate results. Excellent zinc recoveries and concentrate grades have been obtained in pilot testwork using this method. For example, zinc grades of over 65% and recoveries of over 90% have been achieved. However, magnesium that is leached from slag and carried through in solution with zinc reports to the concentrate under the described operating conditions. The resulting magnesium concentration, between 3-4%, is such that the concentrate attracts significant penalties when sold to a zinc refinery. As a result, the process described by U.S. Pat. No. 6,726,889 is not suitable to produce a saleable concentrate from slag leaching solution. The magnesium concentration is such that the zinc oxide concentrate can only be sold with significant financial penalties, making the overall process unfavourable.
The applicant does not concede that the prior arts discussed in this specification forms part of the common general knowledge in Australia or elsewhere.
Throughout this specification, the term “comprising” or its grammatical equivalents are to be taken to have an inclusive meaning unless the context of use indicates otherwise.